The authorities in this case would be the Department of Justice, which until January 20th still reports to Donald Trump.
If tech companies shut down services that are actively supporting an existential threat to the US as a constitutional republic, it seems to me that it's their clear moral and ethical duty to stop providing service to those threats if at all possible.
Furthermore, at this point it's clearly in their self interest to break all ties with Parler, since they open themselves to liability if Parler organizes another successful terrorist attack, now that they know that such activities are being organized there.
If anything, I would argue that they should have policed that content in a much more opinionated fashion far sooner. It's only because things have escalated to the point of violence that it's now so controversial.
Let me give an example to illustrate - suppose that Al Queda, or ISIS, or some other terrorist organization was using AWS or GCP. Would it be so controversial to refuse service to those organizations then? At the very least I don't see why as private businesses they couldn't decide that serving such organizations is too much risk to take on.